Add forward-compatibility glue with pkg-1.3:
 - Use ASSUME_ALWAYS_YES=YES instead of ASSUME_ALWAYS_YES=1
   since pkg-1.3 expects "yes" or "true" values.
 - Before exporting PKG_ABI, strip extra characters from what
   is parsed from 'pkg -vv'.  This causes problems further down
   when creating the packages directory for inclusion on the
   dvd1.iso.  Previously PKG_ABI would be 'freebsd:9:x86:64',
   but now is '"freebsd:9:x86:64";' in pkg-1.3

Tested on:	stable/9@r265858 with ports-mgmt/pkg-devel
MFC After:	3 days
Sponsored by:	The FreeBSD Foundation
Disable the main FreeBSD pkg(7) repositories in the dvd
repository configuration to avoid fetching from upstream
in case there may be conflicts.

MFC After:	3 days
Sponsored by:	The FreeBSD Foundation
Fix syntax error thrown at the point of creating the root pool, caused by
an embedded newline appearing within the options string surrounded by
double-quotes. Rework the logic that goes into setting dataset options on
the root pool dataset while we're here -- added two new variables (which
can be altered via scripting) ZFSBOOT_POOL_CREATE_OPTIONS and also
ZFSBOOT_BOOT_POOL_CREATE_OPTIONS for setting pool/dataset attributes at
the time of pool creation. The former is for setting options on the root
pool (zroot) and the latter is for setting options on the optional separate
boot pool (bootpool) implicitly enabled when using either GELI or MBR. The
default value for the root pool variable (ZFSBOOT_POOL_CREATE_OPTIONS) is
"-O compress=lz4 -O atime=off" and the default value for separate boot pool
variable (ZFSBOOT_BOOT_POOL_CREATE_OPTIONS) is NULL (no additional options
for the separate boot pool dataset).

Reviewed by:	allanjude
MFC after:	7 days
X-MFC-with:	r266107-266109